1. Field of the Invention
The present invention relates to the field of fluorescent drive ballast design and more particularly to the control of the drive voltage applied to a fluorescent lamp load by shifting the drive frequency the ballast driving the lamp load.
2. Description of Related Art
Recently, fluorescent lamps have been used for back lighting of LCD displays, typically in notebooks and other similar consumer applications as well as for military applications including GPS navigational aids. The lamps for such applications are small and are used alone or in combinations of up to four or more lamps depending on the size of the display. Such lamps have a maximum brightness range of 5:1, and their efficiency is slightly more important than for home or office lighting.
In military, industrial and law enforcement applications, LCD displays using fluorescent lamps are found in aircraft cockpits and other high technology applications. Such applications employ one to forty, or more, lamps in combination and represent examples of high-power density applications with 100 watts or more for a single 6xe2x80x3xc3x979xe2x80x3 display. The information displayed on such displays must be visible in direct sunlight and have a dimming range of over 500:1, and they must operate with high efficiency.
Prior art methods for dimming such light arrays typically vary the duty cycle of the AC drive to the lamp, while keeping the drive frequency constant, or they vary the current to the lamp while maintaining a 100% duty cycle.
A first advantage of the present invention is that it allows a wide range of control of the lamp""s brightness, with no discontinuities or steps. The invention also compensates for the effects of temperature on the lamps and components, the effect of aging on the lamps and components, and the effects of input voltage line variations. Furthermore, it can be used in conjunction with other control methods such as pulse width modulation (PWM) to extend the dimming range.
The invention does so with minimal effects on cost, size and efficiency. Existing ballasts may be improved using this invention with no change to the major components, just by changing the way the components are controlled. This control may be handled in a single IC such as a microprocessor, which also incorporates all other control functions and so may not represent a cost increase.
Prior art requires significantly increasing the number of power components, which are large, costly, and waste power.
The invention automatically adjusts the voltage applied to the fluorescent lamp load so as to maintain a constant level of brightness out of the lamp load by sampling the light from the lamp load using an optical detector to develop a brightness signal, peak detecting the brightness signal and developing an error signal by comparing the brightness signal with a reference signal from a reference voltage source. The error signal is then integrated and the integrated error signal drives a voltage controlled oscillator to shift the operating frequency of a ballast drive circuit, as required, to drive the integrated error signal to zero.
In a preferred embodiment, the fluorescent ballast and control circuit comprises a drive signal generator that receives a drive frequency control signal. The drive signal generator provides a first and a second ballast drive signal. Each respective ballast drive signal is phase shifted to insure that they do not overlap in time. Each respective drive signal also has a substantially equal number of volt-seconds and a frequency proportional to the drive frequency control signal.
The fluorescent ballast and control circuit also comprises a fluorescent ballast circuit coupled to the ballast drive signals and having an output voltage coupled to drive a fluorescent lamp load. The fluorescent ballast circuit is characterized to provide a change in the output voltage applied to the lamp load in response to a change in the ballast drive signal frequency. The fluorescent ballast circuit has a transformer with a primary winding and a secondary winding. A totem-pole drive circuit is coupled to drive a first end of the primary winding in series with a resonant inductor. A second end of the primary winding is connected to ground. The secondary winding is connected in parallel with a resonant capacitor and the lamp load.
A means for monitoring the brightness of the lamp load and for developing a brightness signal characterizing the brightness of the lamp load is formed from a photo-cell or photodiode positioned to sense light rays from the lamp load and provide an optical signal characterizing the brightness of the lamp load in response to application of the ballast drive signal or drive pulses to the fluorescent ballast circuit input terminal during an on-time interval.
A signal conditioner is formed from a peak sample and hold circuit coupled to the brightness signal to sample and store the peak value of the brightness signal. The signal conditioner responds to the peak brightness signal and to a reference signal and provides and adjusts the drive frequency control signal to keep the brightness signal substantially constant.
The signal conditioner also has a summing amplifier that has a first input coupled to the reference signal and a second input coupled to be responsive to the peak brightness signal. The summing amplifier scales and outputs the difference between the peak brightness signal and the reference signal from scaled reference voltage source and outputs an error signal. An integrator has an input coupled to integrate the error signal and an output for outputting an integrated error signal. A range limit circuit responsive to the integrated error signal by clamping or limiting the range of the integrated error signal. The range limit circuit outputs the drive frequency control signal.